Three types of polymerization mechanisms are mainly known, i.e. anionic, cationic and radicalar. Generally, monomers bearing double bond-type functions conjugated or activated and allowing propagation of radicalar species are mostly used. It is however difficult to control the molecular weights of the polymer, especially because the radicals are inactivated by oxygen. Recently, monomers bearing vinyl ether-type functions CH2═CHO— with a high cationic polymerization reactivity have been commercialized. Cationic polymerization, though very rapid, hardly leads to high molecular weights mainly because of the fact that the carbocations allowing the propagation of the polymerization are sensitive to water or other nucleophilic species present in the reaction medium.
The compositions mainly used for making films, varnishes or inks generally combine epoxide-type monomers with monomers bearing one or more vinyl ether functions, and are polymerized with cationic catalysts. Because the polymerization of epoxides and the polymerization of vinyl ethers are propagating at very different speeds, the macromolecular solids obtained are usually made of interpenetrated networks corresponding to each type of monomers. The polymerization degree and cross-linking density of such systems are thus hardly controllable. The mechanical properties are more dependent on the rigidity of the chains or the presence of OH functions providing strong hydrogen bonds. These compositions, particularly those to which glycol or triol vinyl diether are added, allow nevertheless the minimization of volatile solvent consumption because of the low viscosity of the corresponding monomers (so-called “reactive diluents”). In that respect, vinyl ethers have a low toxicity.
The “reactive diluent” notion is also used with monomer mixtures of the acrylic type (radicalar) with monomers of the vinyl ether type, in the presence of radicalar initiators added to cationic initiators. It is however as difficult with this procedure to ensure a regular control of the polymerization/cross-linking rate because of the radicalar polymerization sensitivity to oxygen. This problem is particularly true for thin films with a major portion of their surface in contact with air. Polyfunctional monomers containing a vinyl ether function have been described in U.S. Pat. No. 5,605,941. These compounds are used to obtain cross-linked resins having a high glassy transition temperature through a single step process using interpenetrated networks of the (cationic+cationic) or (cationic+radicalar) type.
Anionic polymerization has various advantages in terms of the accurate control of the molecular weight, particularly for a narrow distribution of the weights Mw/Mn. This is the method of choice to prepare polymers with a predetermined weight and elaboration of bloc polymers of the type AB, ABA, or branched. However, the initiators and the anionic species allowing the propagation are highly reactive. These species are either organometallics or alkaline metal alkoxides that react with most organic functions borne by the monomers, particularly those that would allow easy cross-linking later, such as functions containing epoxides, alcohols or amines, or double bonds activated with one or more conjugated double bonds, and aromatic nucleus or an electron attracting group like C═O, C≡N. The water or other nucleophile exclusions conditions during the polymerization cause the polymer to be prepared in dedicated units, rather than at the time of use or moulding.
Also known are polymers having ether functions in high concentration, generally between 40 and 100% molar, particularly containing units —[(CH2H(R)O]n— wherein 4≦n≦2×104 and R is H or an alkyl group of 1 to 4 carbon atoms, or a polymerizable group such as the allyloxy-methyl group. The copolymers, in particular those wherein R is mainly H, possess the property of dissolving certain salts, metallic or onium (ammonium, amidinium, guanidinium) to form conductive solid solutions. Lithium salts are particularly useful to form electrolytes that can be used in primary or secondary batteries, supercapacities, or light modulation systems, also called “electrochromic”. The environment in which these materials work, in particular in contact with highly reductive elements, such as metallic lithium, alloys thereof or solid solutions thereof in the various forms of carbon, like graphite or cokes, requires an increased stability of the bonds of the polymer, that are mainly limited to CH and C—O bonds of the ether functions. Because of the low intrinsic conductivity of these materials, they are embodied in thin films having nevertheless good mechanical behaviour. This is obtained either by using high molecular weights, or more conveniently, through a cross-linking process. The latter method, however, has the disadvantage of increasing the glassy transition temperature (Tg) of the network, which is the most important parameter to determine the conductivity. Further, the allyloxymethyl functions introducing the functions allowing the cross-linking, in particular with the monomer allyl-glycidyl ether (AGE), that are resistant to the action of catalysts for the anionic polymerization, are not very active for the later formation of cross-linking knots, and it is impossible to control exactly the cross-linking density of the materials containing this monomer. It is difficult to implicate more that 50% of the double bonds during the cross-linking.
Polymers obtained from oligo(ethylene oxide) vinyl ethers have been proposed as solid electrolytes, for example in U.S. Pat. No. 4,886,716, U.S. Pat. No. 5,064,548, U.S. Pat. No. 5,173,205, U.S. Pat. No. 5,264,307, U.S. Pat. No. 5,411,819 and U.S. Pat. No. 5,501,920, and possess acceptable conductive properties. The preparation of the corresponding monomers is however delicate. These materials, before cross-linking, have a low molecular weight, as for most of cationic polymerizations. Indeed, the monomers are highly hygroscopic, and thus difficult to purify. In addition, the mechanical properties of the polymers are poor because of the absence of entanglement, a phenomenon typical to polymers with lateral chains. By using polyfunctional monomers of the vinyl diether-type, it is possible to obtain cross-linked products, but it is however impossible to separate the polymerization step from the cationic cross-linking process, and as a result, there is substantially no control of the process for obtaining the cross-linked polymer.